MicroRNAs (miRNAs) are short, non-protein coding RNAs that bind to near complementary RNA sequences to inhibit gene expression. The control of gene expression by miRNAs has emerged as an important mechanism for gene regulation in a wide range of organisms (flies, worms, plants, and mammals) as well as a potential tool for treating disease. MiRNAs have recently been demonstrated to play an important role in the regulation of pathological conditions in humans related to cell death, viral infection, insulin secretion and cancer. Furthermore, it is estimated that over 30% of all human genes are regulated by miRNAs. MiRNAs regulate many aspects of plant development. Among these is the process of developmental phase change. Early in their development, Arabidopsis plants produce morphologically simple juvenile leaves. Later, they undergo a transition known as vegetative phase change and begin to produce more complex adult leaves. Finally, the plants produce flowers, marking the final stage of plant development. The miRNA, miR156, is a major regulator of this process. Using the experiments described in this proposal, I will define the factors that contribute to the temporal expression of miR156. Specifically, I will: AIM 1: Determine if miR156a temporally regulates SPL expression using mutant alleles of the miR156 and its targets. AIM 2: Develop reporters for use in promoter-deletion studies and mutant screens to identify cis- and transfactors involved in the temporal regulation of miR156a. AIMS: Identify genes that regulate miR156 expression by screening for suppressors and enhancers of sqn. The results of these studies will not only contribute to the understanding of leaf development, but will also increase our understanding of the mechanism of temporal regulation, a key process in developmental biology of all organisms.